1. Field of the Invention
This invention pertains to injection molding, and more particularly to apparatus associated with parts produced by an injection molding machine.
2. Description of the Prior Art
It is well known to make parts from thermosetting plastic materials on injection molding machines. Liquid plastic material is pumped at very high speed and pressure into a cavity of the proper shape and size. The cavity is defined by surfaces of two molds that reciprocate toward and away from each other. When the molds are closed against each other, the cavity is closed to accept the plastic material. After the plastic has solidified, the molds open. An ejector mechanism acts to remove the part from the molds. The cycle then repeats.
In a prior design, one mold was mounted directly to a platen of the injection molding machine. The other mold was fastened to an ejector housing. The ejector housing, in turn, was mounted to another machine platen. Although it was rather simple, the prior design had the great disadvantage of being extremely costly to make. Because of the high speeds and pressures associated with injection molding, the molds were invariably made of very strong material, such as heat treated steel. The steel material was very expensive to machine in order to make the surfaces that formed the cavity when the molds were closed. Not only did the molds represent a large investment, but also the molds could be used for only a single part. Moreover, the investment for a particular part was idle when the molds for that part were not in use.
A closely related problem concerned the prior mechanisms for ejecting parts from the molds. The ejector mechanisms included a series of ejector pins held on an ejector plate. The ejector pins were located specifically to suit the particular molds and the part to be made. The entire assembly thus represented a very substantial investment that was custom designed and constructed to suit a single part.
In an alternate prior design, the two molds were relatively small in size and were used with respective clamping or support plates. Although the cost of the material was less for the molds, the cost of machining them remained high. In addition, the ejector plate and pins had to be custom made to suit the particular part.
Another disadvantage of prior mold assemblies concerned cooling them. The molds were drilled for cooling water passages in a manner that best suited the particular part being molded. Drilling long holes in the steel molds added even more expense to the mold assembly. Further, the locations of the fitting ports on the mold exterior frequently were at inconvenient locations relative to the rest of the injection molding machine.
Thus, a need exists for improvements to injection molding machine mold assemblies.
In accordance with the present invention, a mold base system for an injection molding machine is provided that is far more economical and easier to use than prior equipment. This is accomplished by apparatus that includes a pair of mold bases each having several lugs that accurately hold an insert that forms a part producing cavity.
In keeping with the economical nature of the invention, the mold bases are cast. Each has four outside walls and opposed edge surfaces. The mold base has a floor with a central opening in it. Upstanding from the floor are the lugs. The lugs are arranged to make the corners of a geometrical configuration, such as a rectangle. A series of ribs connect the four outside walls to the lugs. The ribs are arranged to make pockets between them and the outside walls.
The lugs of each mold base are machined to accurately but removeably hold an insert. The insert associated with one mold base is contoured to form half of a cavity that is the negative of the part to be made on the injection molding machine. The insert associated with the other mold base is contoured to form the other half of the cavity.
It is an important feature of the invention that access to the insert cooling water ports is available from all sides of the insert. First fittings are inserted into the insert ports, and other fittings are connected to ports on the inside of the desired mold base wall. Tubes connect the fittings in the insert and mold base wall. To easily accommodate the tubes, the mold base ribs may be notched. In that manner, the water lines from the insert can be very easily routed to any desired wall of the mold base without being limited by the arrangement of the cooling ports in the insert. Lines connected to ports on the outside of the mold base wall lead to a pump, chiller, and other known equipment for cooling the water.
The outstanding advantage of the mold base system of the invention is that only the inserts need to be switched when molding different parts. The mold base can remain on the injection molding machine. To remove an insert, it is necessary merely to disconnect the water lines and a few screws that hold the insert to the mold base. Further, the cost of the material of the relatively small insert is much less than the cost of the entire prior mold that was replaced previously when making a new part. In addition, the cost associated with drilling the cooling holes in the prior large molds is greatly reduced.
Further in accordance with the present invention, the same cast ejector housing is usable with all the different inserts that can be used with the mold base. Consequently, the ejector housing remains permanently fastened to the mold base. The ejector housing has a number of pillars on a central wall that support both the insert and the mold base lugs and ribs. The ejector housing central wall also has several holes through it that receive knockout pins in contact with an ejector plate. The ejector plate is slidingly guided on a pair of columns upstanding from the ejector housing central wall.
In the preferred embodiment, a cap is incorporated into the ejector plate in a manner that enables the ejector plate to accommodate a wide variety of patterns for the ejector pins. The ejector plate cap is in alignment with the insert in the mold base. The cap has a counterbored hole in alignment with each of the ejector holes in the insert. A head of each ejector pin is captured between the corresponding counterbore in the ejector plate cap and a surface on the ejector plate. When switching inserts to mold a different part, it is an easy task to remove the ejector plate cap from the ejector plate and replace it with a different cap that has the new pattern of ejector pins required for the new insert.
Because of the very large forces that are produced during the molding process, it is important that the components of the mold base system be rigid. In particular, the inserts must remain stably in place during a molding cycle. To increase the rigidity of the mold base system, a series of blocks can be placed between the insert and the mold base walls and ribs. The blocks fit accurately and snugly between the insert and the mold base such that they transfer the forces of the insert to the mold base and thus rigidly retain the insert in place. To suit the cooling lines between the insert and the mold base, the blocks can be cut out in appropriate patterns.
Other advantages, benefits, and features of the present invention will become apparent to those skilled in the art upon reading the detailed description of the invention.